1. Field of the Invention
The present invention relates to a propulsion system for a magnetically levitated vehicle and, more particularly, to the construction of the fixed propulsion coils which interact with the superconducting coil mounted on the levitated vehicle so as to propel the vehicle.
2. Description of the Prior Art:
FIG. 5 is a plan view showing the positional relationship between the superconductor coil mounted on the levitated vehicle, identified as 1, and the propulsion coil mounted on the ground, identified as 4. The propulsion coil shown at 4 is a three phase coil wherein each coil is located in the same plane as every other coil, this design propulsion is disclosed in document LD-89-27 of Linear Drive Research Division, Society of Electronic Engineers.
The superconductor coil 1 may be excited at either the north or south poles, more particularly, the superconducting coil are excited at the north and south poles which are arranged alternatingly at a pole pitch l.sub.tp, thus forming the field poles of the synchronous motor. The propulsion coil 4 further has coils of three phases U, V and W which are arranged at a pitch l.sub.rp which is two-third the pitch l.sub.tp of the superconducting coil 1, thus forming the armature of the linear synchronous motor.
An alternative prior art arrangement is illustrated in FIG. 6 which is the plan view showing the positional relationship between the superconductor coil mounted on a vehicle and the propulsion coil mounted on the ground. In this embodiment, the propulsion coils comprise a double-layered three phase coil as disclosed in document LD-89-27 of the Linear Drive Research Division, Society of Electronic Engineers, referred to previously. In FIG. 6, item 14 denotes the propulsion coil which is a double-layered structure composed of an inner coil 14a and an outer coil 14b. The inner coil 14a has a length which is equal to the pole pitch l.sub.tp of the superconductor coil 1 and arranged at a pitch which is 4/3 l.sub.tp. The outer coil 14b has the same construction as the inner coil 14a. The same phases of both coils 14a and 14b are arranged at a pitch represented by 2l.sub.tp.
In the single layer propulsion coil is employed three phase electrical power of a phase and frequency synchronous with the position and speed of the vehicle is supplied to the propulsion 4 from a variable-voltage, variable-frequency power supply which is not illustrated. As a consequence, a propulsion force is generated which acts on the superconductor coil 1 on the vehicle to propel the vehicle forward at the desired speed. When this propelling relationship is viewed from the vehicle, an electromagnetic pulsation having a main pulsating component of a wavelength corresponding to the pitch of the propulsion coil 4 is generated which electro-magnetically vibrate the superconductor coil 1 on the vehicle. Since the three coils of U, V and W of the propulsion coil 4 are arranged between a pair of poles on the superconductor coil 1, the electromagnetic pulsation forms a frequency of the third order with the main component of the electromagnetic pulsation being caused by the space higher harmonic magnetic field components of the second order, with respect to the fundamental frequency of the propulsion current, wherein this may be represented by the formula: F.sub.1 =V/2l.sub.tp, wherein V represents the speed of the vehicle in meters per second, l.sub.tp represents the pole pitch in meters of the superconductor coil.
The pulsating magnetic force causes the on-board superconducting coil to oscillate, i.e., it mechanically vibrates. This vibration causes the cryogenic container to vibrate with the superconductor coil. Such vibrations are transmitted to the remainder of the vehicle to create undesired vibrational effects. In addition, an eddy current may be generated in the container holding the superconduction coil which can cause the coolant, normally liquid helium, to vaporize to cause still further difficulties. In addition, the vibration is amplified when it is in a resonant condition as the car speed increases. It is well known that in magnetically levitated vehicle propulsion when one has a single layer three phase coil, the electromagnetic pulsation acts on the superconductor coil 1 as a progressive wave, which excites the superconductor coil with a frequency which is proportional to the vehicle speed.
The other coil arrangement illustrated in FIG. 6 for propelling the magnetically levitated vehicle employs a two-layered three phase coil. The use of the two-layered three phase coil can significantly reduce the second order space higher harmonic magnetic fields generated in the propulsion system using the single layer three phase coil which thereby substantially eliminates the vibration of the superconducting magnetic coil. When using the two-layered coil, the main space higher harmonic magnetic field is of the fifth order and frequency is of the sixth order. The magnitude of the space higher harmonic component usually decreases as the order number increases. Consequently, the utilization of a two-layered coil in the propulsion system may substantially decrease the magnitude of the electromagnetic pulsation acting on the superconducting coil 1 during the operation of the vehicle as compared with the propulsion system employing only a single layered three phase coil.
However, the solution offered by the arrangement of FIG. 6 introduces a new problem, electromagnetic pulsation of the pitch of the propulsion coil 14 of the same layer which is 4/3l.sub.tp, i.e., an electromagnetic pulsation caused by a space higher harmonic magnetic field of 0.5 order acts on the superconductor coil 1. The electromagnetic pulsating force has a frequency of the 1.5 order and comprises as much as 26% of the propulsion force in terms of left and right magnetic force components which are considered to be the components producing the worst effect on the superconductor coil 1. As a consequence, a pulsating magnetic force in the lateral direction is generated which also induces undesired vibration as well as reduced energy efficiency.
Accordingly, a need continues to exist for minimizing pulsating magnetic forces in magnetically levitated vehicles propelled by linear motors. In particular, a need continues to exist for a linear motor which is substantially free of vibrations caused by third harmonic and the 1.5 harmonic.